This invention comprises two alternate mounting methods and devices to secure a variety of measuring, musical, telecommunications, audio, videorecording and similar devices to mounting rails equipped with square pitch-holes in either standardised or otherwise regular attachment. Typically mounting devices comprise bolts securing the front plates of sub-racks to cage nuts installed in the square-pitch holes in the mounting rails. As a result of standardised or regular pitch-hole arrangement mounting locations are limited. Alternate mounting methods and devices described here will increase the amount of mounting locations.
The use of mounting rails has arisen from the need to combine together different self-contained pieces of equipment by housing them in sub-rack cases. Mounting rails have then been used to build cabinets, to which pieces of equipment have been installed by securing them from their front plates to the mounting rails. Cover-plates and parts needed for structural integrity of the cabinet have typically also been secured to the mounting rails. Early mounting rails were typified by holes equipped with grooves drilled to solid metal rails, but they gave way to solutions like the cage-nut, allowing bigger tolerances of measure, choice of grooves to be used and easier maintenance in case of breakage.
There are also known from the past mounting methods based on receptacles for mounting bolts that move freely along the total length of the mounting rail. System such as this allows securing of sub-racks to any height chosen in the mounting rail. However, securing receptacles that are allowed to move freely vertically are unreliable with heavy loads, as securing is based on friction or similar arrangement. Horizontal straightness of sub-racks to be mounted is not easy to achieve and their distances from each other doesn't remain constant as new sub-racks are added in the course of time. Freely adjustable mounting has usually been used only in lightweight assemblies or in panel- or sub-assemblies within sub-racks themselves. The profile structure used in freely adjustable mounting rails makes it an expensive solution.
Mounting rails with fixed pitch-holes arranged according to standard also pose problems. Due to standards widely used by rack-cabinet manufacturers the mounting-holes of equipment front plates correspond to the size of the equipment unit U (U=unit). Fixed in width, the size of equipment varies in height, expressed in sequence of U's as in 1U, 2U, 3U, etc. Exact directives regulating rack-rail pitch-hole and front plate mounting hole arrangement can be found in standards. Most common arrangement can be found for example in IEC standard 297.
Due to the combined effects of this system of mounting and standardisation the space needed for cable pass-throughs and ventilation between the sub-racked pieces of equipment always requires at least 1U of empty space to be left. Leaving an in-between space of at least lU is also required when mounting pieces of equipment that don't conform to standard heights, or devices that have standing-feet for free-standing operation. The required amount of space to be left empty in a rack-cabinet varies according to specific needs of different groups of end-users.
Practice has shown, that due to standardisation, in extreme cases only every second 1U space available in a mounting rail can be used. This is specifically true when four-point mounting (securing the device from all of it's front plate's four corners) is used. Securing the device only from its two lowest front plate mounting-holes allows it to be secured to mounting rail pitch-holes intended to secure front plates upper portion, but because of the nature of pitch-hole sequence this approach is only valid for one particular piece of equipment, as the next device immediately above would have to be secured only from its uppermost front plate mounting-holes. This would result in a highly unstable mounting. No significant advantage or variation can be gained by applying two-point mounting to an EIC regulated mounting rail in an effort to reduce the size of empty space between devices. Two-point mounting adds stress to mounting structures and is totally unacceptable in mobile installations.
To add variation to the mounting possibilities allowed by IEC standard, an additional pitch-hole of fixed type has sometime been added exactly in the middle of each space designed to receive 1U in the mounting rail. Using this additional pitch-hole it is possible to randomly create smaller gaps between units to be mounted, but all but 3U high devices and certain specially formatted devices can only be two-point mounted in this position. As such, these pitch-holes do not constitute a significant improvement to the standardised arrangement.